We propose to develop genetic procedures for studying gene expression, structure, and function through the use of gene fusions, and to use these procedures to study in depth the transposition of the Tn3 DNA transposable element. These procedures will permit the detection, isolation, and alteration of gene control signals by fusing with complementary gene segments from well-characterized genes such as the lactose operon B-galactosidase gene. Fusions will be constructed by joining DNA segments either in vitro with restriction endonucleases and DNA cloning, or in vivo with semi-site specific recombination of transposable elements such as bacteriophage Mu. The application of these methods to Tn3 involves the characterization of the genetic structure of the Tn3 element and the elucidation of the functions of the Tn3 gene products and their interaction with chromosomal gene products. One promising approach, for example, has been to use Tn3 transposase-B-galactosidase hybrid protein gene fusions to isolate mutants, both on Tn3 and on the chromosome, which overproduce the transposase protein and result in a higher frequency of transposition. These mutants will be used to purify and biochemically characterize the transposase protein and its possible interaction with chromosomal gene products. The experiments described here for studying Tn3 are designed to be generally applicable to other genetic systems both in E. coli and in other species for which DNA transformation is possible.